1. Research Scientist, Virginia Transportation Research Council, Charlottesville, VA 22903 (corresponding author).
2. Deputy Director of Research, Virginia Transportation Research Council, Charlottesville, VA 22903.
3. Environmental Lab Manager and Lecturer, Dept. of Engineering Systems and Environment, Univ. of Virginia, Charlottesville, VA 22904.
4. Professor, Dept. of Engineering Systems and Environment, Univ. of Virginia, Charlottesville, VA 22904.